The success of the Colorado ACTU in performing studies of antiretroviral therapy depends on the close support of a basic virology laboratory. The Virology Core Laboratory of the Colorado ACTU has fulfilled this purpose since establishment of our subunit in August, 1990 and is certified by the ACTG Virology Committee in the performance of HIV-1 cultures and serum p24 antigen assays. A proposal is therefore submitted to provide virology core laboratory support to the Colorado ACTU Clinical Core and to other cooperating ACTU's as outlined in the RFA. In addition to serum p24 antigen determinations and qualitative HIV-1 culture, our laboratory can perform plasma viremia assays, quantitative HIV-1 microcultures, zidovudine resistance assays, and amplification of HIV-1 DNA by polymerase chain reaction for genotype analysis of RT. Additional techniques for quantifying viral load and assaying antiretroviral drug susceptibility are being developed or will be adopted as they become available.